Solder mask compositions for aerosol jet printing

ABSTRACT

A solder mask ink for aerosol jet printing includes a metal oxide and a propylene glycol-based solvent; the solder mask ink has a viscosity from about 50 cps to about 1,000 cps at shear rate of 10 1/s at 25° C., and a shear thinning index from about 1.0 to about 2.0.

BACKGROUND

Embodiments disclosed herein relate to solder masks such as thoseemployed in the manufacture of printed circuit boards. In particular,embodiments disclosed herein relate to solder mask inks having suitableviscosity for aerosol jet printer application.

Printed circuit boards (PCBs) or printed wiring boards (PWBs)(hereinafter collectively PCB's) are platforms that connect andinterface electronic components with each other and with other elementsin computers, communication devices, consumer electronics, automatedmanufacturing and inspection equipment. PCB's may be produced from abase substrate, typically an insulating material, on which a thin copperlayer is laminated or plated. Chemical etching is then used to removeareas of the copper to produce electrically conducting paths or traces.The traces permit electrical interconnection of the components attachedto the PCB.

An insulative material, referred to as a solder mask, is then appliedover the copper conducting paths. Solder masks protect the conductingpaths on the PCB from being coated with solder during soldering steps,while leaving uncovered only the conducting pads that need to becontacted with molten solder. The solder mask layer on simple PCBs maybe produced using screen-printing or spin-casting techniques. However,more densely populated PCBs typically utilize lithographic techniques toform a patterned solder mask on the copper layer.

Lithographic techniques used to prepare solder masks can involvemulti-step sequences that are material and energy-intensive. Forexample, the process usually involves film coating, lithography, wetetching and curing, as indicated in the flow diagram of FIG. 1. In sucha process, the solder masks are often epoxy-based materials that arespin-coated or applied in an analog fashion, followed by subtractiveetching. The process tends to deteriorate the chemical and physicalresistance of the final cured solder mask. Because the first step ofphotolithography is non-selective, via holes in PCB are often partiallyor completely filled with solder mask. Removing the solder mask for highaspect ratio via holes is a very difficult and sometimes impossibletask. Ultimately, such methods can be a costly and wasteful.

Although digital methods for depositing solder masks is desirableattempts at inkjet printing, for example, have run up againstlimitations due to the very low viscosity requirements (less than about20 cps). By contrast, commercial solder mask resists typically have veryhigh viscosities (greater than about 10,000 cps), and thus, inkjetprinting solder mask systems are difficult to put into practice.

Alternatively, screen printing has been developed for depositing soldermask resist. Although it can overcome the via hole plugging issue,screen printing often yields solder masks with low resolution and poorregistration. In addition, screen printing requires a flat surface. PCBswith relief structures on the surface are not amenable to screen printsolder mask thereon. Similarly, solder masks generally cannot be screenprinted on curved surfaces or on 3D electronics.

SUMMARY

In some aspects, embodiments herein provide solder mask inks for aerosoljet printing comprising a metal oxide and a propylene glycol-basedsolvent, wherein the solder mask ink has a viscosity from about 50 cpsto about 1,000 cps at shear rate of 10 1/s at 25° C., and a shearthinning index from about 1.0 to about 2.0.

In some aspects, embodiments herein provide solder mask inks comprising:i) a resin or UV curable monomer; ii) an inorganic pigment; and iii)from about 20 to about 50 percent by total weight of the ink of an etheror an ester solvent based on propylene glycol, wherein the solder maskink formulation has a viscosity from about 50 cps about 800 cps at shearrate of 10 1/s at 25° C., and a shearing thinning index from about 1.0to about 1.5.

A method comprising aerosol jet printing a solder mask ink in a patternon a substrate, the solder mask ink comprising: i) a resin or UV curablemonomer; ii) an inorganic pigment; and iii) about 20 to about 50 percentby weight of an ether or an ester solvent based on propylene glycol,wherein the solder mask ink has a viscosity from about 50 cps to about800 cps at shear rate of 10 1/s at 25° C., and a shearing thinning indexfrom about 1 to about 1.5, and curing the jetted solder.

BRIEF DESCRIPTION OF DRAWINGS

Various embodiments of the present disclosure will be described hereinbelow with reference to the figures wherein:

FIG. 1 shows a conventional solder mask deposition process.

FIG. 2 shows a dual plot of viscosity (at 25° C.) and shear thinningindex of an exemplary solder mask in accordance with embodiments hereinas a function of solvent addition. The shear thinning index is the ratioof viscosity at low shear of 10 1/s to the viscosity at high shear of484 1/s.

FIG. 3 shows a scratch resistance and adhesion test of an exemplarysolder mask in accordance with embodiments herein formulated with acombination of propylene glycol monomethyl ether acetate and dipropyleneglycol monomethyl ether acetate (PGMEA/DPGMEA). No material wastransferred to the tape, indicating an excellent adhesion of 5B. Thepencil hardness was 6H, the highest rating for a solder mask.

FIG. 4A shows an image of a cured exemplary solder mask in accordancewith embodiments herein printed on polyethylene terephthalate (PET)substrate at different speeds (5.0 mm/s, 10 mm/s and 20 mm/s).

FIG. 4B shows an optical image of a line printed at 10 mm/s from FIG.4A; the line exhibits smooth edges.

DETAILED DESCRIPTION

Embodiments herein provide solder mask inks suitable for aerosolprinting. The solder mask inks generally have viscosities less thanabout 1000 cps at shear rate of about 10 1/s at 25° C., and a shearthinning index less than about 2.0. In embodiments, the solder mask inksmay comprise: i) resin or UV curable monomers; ii) an inorganic pigment,and iii) at least about 20 weight percent of the total weight of theink, and up to about 50 weight percent of an ether or ester solventbased on propylene glycol, the solder mask inks having a viscosity lessthan about 800 cps at shear rate of about 10 1/s at 25° C., and ashearing thinning index less than about 1.5. Such solder mask inksdemonstrated good printability in aerosol jet printer, and the cured,printed masks exhibited comparable adhesion, pencil hardness, scratchresistance, and chemical resistance as commercial solder masks.Advantageously, the use of aerosol printing of the solder mask inksherein allows for placement of solder mask patterns on irregularlyshaped surfaces, in sharp contrast to conventional solder maskformulations.

As used herein, “shear-thinning index” or “STI” is a unitlessmeasurement proportional to the ratio of viscosities of a fluid measuredat a low and high speed. In embodiments, the STI is defined as the ratioof viscosity at the shear rate of 10 1/s over the viscosity at the shearrate of 484 1/s at 25° C. Thus, the shear-thinning index is aspeed-dependent viscosity ratio. Those skilled in the art will recognizethat the STI is sometimes also referred to as a thixotropic index, butthis should not be construed as thixotropy.

Without being bound by theory, the benefits of the disclosed solder maskinks may be realized via the high solids content and specific rheologyand shear thinning behavior allowing delivery by aerosol jet. Inparticular, the selection of propylene glycol-based solvents not onlydecreases the viscosity, but also reduces the shear thinning index ofthe solder mask inks, thus effectively reducing viscosity at relativelylow concentrations. Indeed, dilution of solder mask base materials toaerosol jettable viscosities alone is not sufficient and alternativesolvents that provide the requisite viscosity range can cause depositionof very thin layers with pinholes which are inadequate to function assolder masks. Thus, the particular solvents disclosed herein provide notonly a desirable viscosity, but also proper shear thinningcharacteristics to avoid problematic thin layers with pinholes.

Aerosol jet printing with the solder mask inks herein has several otheradvantages: (1) It is a digital process which significantlysimplifies/reduces the process steps, thus reducing the manufacturingcost; (2) Solder mask resists are applied digitally onto the desiredarea, which reduces materials waste and avoids via holes plugging; (3)Aerosol printing has been demonstrated for printing high resolution(e.g., about 10 micron) making it suitable for high-density solder maskmanufacturing; (4) Aerosol printing can handle much higher ink viscosity(up to about 1000 cps), when compared to ink-jet printing; and (5)Aerosol printing has been demonstrated as a suitable method for printingonto 3D surfaces or surfaces with 3D topography relief structures. Theseare all desirable properties for PCB manufacturing and printing 3Delectronics.

In embodiments, the solder mask inks can employ metal oxide-based orother inorganic pigments, in conjunction with the particular enumeratedsolvents, to provide solder mask inks that do not require the use ofsurfactants. This is particularly beneficial to realize good performancecharacteristics of the cured masks, where surfactants can cause someperformance deterioration of the resultant mask. In embodiments, whitesolder mask base materials comprising a metal oxide, such as titaniumdioxide, may be particularly suitable for PCB for LED applications.

In embodiments, there are provided solder mask inks for aerosol jetprinting comprising a metal oxide and a propylene glycol-based solvent,wherein the solder mask ink has a viscosity from about 50 cps to about1,000 cps at shear rate of 10 1/s at 25° C., and a shear thinning indexfrom about 1.0 to about 2.0.

As used herein, a “solder mask ink” refers to compositions that aresufficiently fluid to be deposited by aerosol jet printing and which areemployed in the formation of solder mask patterns. The solder mask inkcan be applied to the copper traces of a printed circuit board (PCB) forprotection against oxidation and to prevent solder bridges from formingbetween closely spaced solder pads. Solder masks are particularly usefulin high throughput PCB board production. The solder mask inks may bebased on epoxy liquids of appropriate viscosity and shear thinning indexfor application via aerosol printing methods. The solder mask inks willtypically employ either a thermal or UV curable resin.

As used herein, “aerosol jet printing” refers to a process thattypically involves atomization of the solder mask ink, which can beoptionally heated, producing droplets on the order of one to two micronsin diameter. The atomized droplets are typically entrained in a gasstream and delivered to a print head. At the print head, an annular flowof gas is introduced around the aerosol stream to focus the dropletsinto a tightly collimated beam. The combined gas streams exit the printhead through a converging nozzle that compresses the aerosol stream to asmall diameter which may be from about 1 micron to about 10 microns. Thejet exits the print head and is deposited on a substrate. The resultingpatterns can have features ranging from about 5 microns to about 3000microns wide, with layer thicknesses from tens of nanometers to about 25microns, including from about 1 micron to about 20 micron.

As used herein, “shear thinning index” refers to a speed-dependentviscosity ratio. It can be obtained by taking a ratio of a viscositymeasured at a low speed at 10 1/s, over the viscosity measured at a highspeed, at 484 1/s. In an ideal Newtonian fluid, the shear thinning indexis close to about 1.0.

In embodiments, the solder mask inks comprise a resin or UV curablemonomer. Such resins or UV curable monomers may include, withoutlimitation, a bisphenol A epoxy, a novolak epoxy, an acrylic acidmodified epoxy, a cycloaliphatic- or heterocycle-based epoxy, andcombinations thereof, the resin being crosslinkable with a phenol,amine, or anhydride. In embodiments, the resin or UV curable monomer maybe provided in a solder mask base material, such as commerciallyavailable solder mask pastes and the like distributed by Taiyo, America,including those under the product line PSR-4000.

In embodiments, there are provided solder mask inks comprising: i) aresin or UV curable monomer; ii) an inorganic pigment; and iii) fromabout 20 to about 50 percent by total weight of the ink of an ether oran ester solvent based on propylene glycol; wherein the solder mask inkformulation has a viscosity from about 50 cps to about 800 cps at shearrate of 10 1/s at 25° C., and a shearing thinning index from about 1.0to about 1.5.

Advantageously, the solder mask inks herein may be formulated withoutsurfactants. Thus, in embodiments, the solder mask is free ofsurfactant. In some embodiments, solder mask inks may comprise an amountof surfactant that is sufficiently low to have little impact on thephysical properties of the cured solder mask. Where surfactants areemployed, they may be non-ioinic surfactants. Examples of non-ionicsurfactants include polysorbates such as polysorbate 20 (polyoxyethylene(20) sorbitan monolaurate), polysorbate 40 (polyoxyethylene (20)sorbitan monopalmitate), polysorbate 60 (polyoxyethylene (20) sorbitanmonostearate), polysorbate 80 (polyoxyethylene (20) sorbitanmonooleate); polyglycerol polyricinoleate, Octadecanoic acid[2-[(2R,3S,4R)-3,4-dihydroxy-2-tetrahydrofuranyl]-2-hydroxyethyl] ester,Octadecanoic acid[(2R,3S,4R)-2-[1,2-bis(1-oxooctadecoxy)ethyl]-4-hydroxy-3-tetrahydrofuranyl]ester; C8 to C22 long chain alcohols such as 1-octadecanol, Cetylstearylalcohol, Hexadecan-1-ol and cis-9-octadecen-1-ol; substituted orunsubtituted octylphenol in which the substituents can include apolyethoxyethanol group (e.g., to form octylphenoxypolyethoxyethanol) orany other substituent that will form a non-ionic surfactant withoctylphenol; Polyethylene glycol monoisohexadecyl ether; Dodecanoic acid2,3-dihydroxypropyl ester; glucosides such as lauryl glucoside,octylglucoside and decyl glucoside; fatty acid amides such as cocamidediethanolamine and cocamide monoethanolamine; and nonionic surfactantsthat have a hydrophilic polyethylene oxide chain and an aromatichydrocarbon lipophilic or hydrophilic group, such as Nonoxynol-9 andTriton X-100.

In an embodiment, the non-ionic surfactant is a polyalkylene glycol. Forexample, the non-ionic surfactant can be a block co-polymer comprisingat least one polyethylene glycol block and at least one polypropyleneglycol block, such as polyethylene glycol-block-polypropyleneglycol-block-polyethylene glycol or a triblock copolymer composed of acentral hydrophobic chain of polyoxypropylene (poly(propylene oxide))flanked by two hydrophilic chains of polyoxyethylene (poly(ethyleneoxide)). An example of a commercially available non-ionic surfactant isSYNPERONIC F108, available from Aldrich.

The non-ionic surfactant can be employed in any suitable amount, forexample, the amount of non-ionic surfactant can be about 0.01% by weightrelative to the total weight of the solder mask ink composition. Inother examples, the amount of non-ionic surfactant can range from about0.05% to about 5%, such as about 0.5% to about 3% by weight relative tothe total weight of the ink composition.

In embodiments, the solder mask inks employ a solvent is an ether and/oran ester solvent based on propylene glycol. In some such embodiments,the solvent is propylene glycol monomethyl ether acetate, dipropyleneglycol monomethyl ether acetate or combinations thereof. When used incombination the solvent may be a 1:1 combination of propylene glycolmonomethyl ether acetate and dipropylene glycol monomethyl etheracetate. In embodiments, solvents comprises about 10 to about 50 weightpercent of the solder mask ink, including from about about 10 to about35 wt % for the solder mask ink. In embodiments, the ratio may be fromabout 9:1 to about 1:9, including from about 7:3 to about 3:7.

In embodiments, solder mask inks may comprise a metal oxide that is aninorganic pigment. In embodiments, the metal oxide is an oxide oftitanium. In embodiments, the solder mask ink may comprise any inorganicpigment, including any inorganic metal-based pigments. Aluminum pigmentsmay include, without limitation, Ultramarine (PB29), a complex naturallyoccurring pigment of sulfur-containing sodio-silicate(Na₈₋₁₀Al₆Si₆O₂₄S₂₋₄), Ultramarine violet (PV15) a silicate of sodiumand aluminum containing sulfur. Copper pigments may include, withoutlimitation, Scheele's Green: copper arsenite CuHAsO₃,Cu(C₂H₃O₂)₂.3Cu(AsO₂)₂, Paris Green: copper(II) acetoarsenite, EgyptianBlue, a synthetic pigment of calcium copper silicate (CaCuSi₄O₁₀), HanBlue: BaCuSi₄O₁₀, Han Purple: BaCuSi₂O₆. Cobalt pigments may include,without limitation, Aureolin (also called Cobalt Yellow) (PY40):Potassium cobaltinitrite (Na₃Co(NO₂)₆, Cobalt Blue (PB28) and CeruleanBlue (PB35): cobalt(II) stannate, Cobalt Violet: (PV14) cobaltousorthophosphate. Manganese pigments may include, without limitation,Manganese violet: NH₄MnP₂O₇ (PV16) Manganese ammonium pyrophosphate,Iron pigments may include, without limitation, Iron black (PBk11) (C.I.No. 77499):Fe₃O₄, Yellow Ochre (PY43): a naturally occurring clay ofhydrated iron oxide (Fe₂O₃.H₂O), Prussian Blue (PB27): a syntheticpigment of ferric hexacyanoferrate (Fe₇(CN)₁₈). Venetian Red, Oxide Red(PR102), Red Ochre (PR102): anhydrous Fe₂O₃, Burnt Sienna (PBr7): apigment produced by heating Raw Sienna, Clay earth pigments (naturallyformed iron oxides), Raw Umber (PBr7): a natural clay pigment consistingof iron oxide, manganese oxide and aluminum oxide:Fe₂O₃+MnO₂+nH₂O+Si+AlO₃. When calcined (heated) it is referred to asBurnt Umber, Raw Sienna (PBr7): a naturally occurring yellow-brownpigment from limonite clay. Cadmium pigments may include, withoutlimitation, Cadmium Orange (PO20): an intermediate between cadmium redand cadmium yellow: cadmium sulfoselenide, Cadmium Yellow (PY37):cadmium sulfide (CdS), Cadmium Red (PR108): cadmium selenide (CdSe),Cadmium Green: a light green pigment comprising a mixture of CadmiumYellow (CdS) and Viridian (Cr₂O₃), Chromium pigments may include,without limitation, Chrome Orange: a naturally occurring pigment mixturecomposed of lead(II) chromate and lead(II) oxide. (PbCrO₄+PbO), ChromeYellow (PY34): natural pigment of lead(II) chromate (PbCrO₄), Chromegreen (PG17): chromium oxide (Cr₂O₃), Viridian (PG18): a dark greenpigment of hydrated chromium(III) oxide (Cr₂O₃), Arsenic pigments mayinclude, without limitation, Orpiment natural monoclinic arsenic sulfide(As₂S₃). Lead pigments may include, without limitation, Cremnitz White(PW1): basic plumbous carbonate ((PbCO₃)₂.Pb(OH)₂), Red Lead: leadtetroxide, Pb₃O₄, Naples Yellow (PY41). Titanium pigments may include,without limitation, Titanium White (PW6): titanic oxide (TiO₂), TitaniumBlack, Titanium Yellow (PY53). Tin Pigments may include, withoutlimitation, Mosaic gold: stannic sulfide (SnS₂). Mercury pigments mayinclude, without limitation, Vermilion (PR106), Mercuric sulfide (HgS).Antimony pigments may include, without limitation, Antimony White:stibous oxide (Sb₂O₃), Barium pigments may include, without limitation,Barium sulfate (PW5), Zinc pigments may include, without limitation,Zinc White (PW4): Zinc Oxide (ZnO).

In embodiments, the inorganic pigment is selected from the groupconsisting of a metal oxide, metal sulfate, metal sulfide, metal borate,azurite, aureolin, cobalt blue, chrome yellow, cerulean, calciumchromate, bice cobalt green, Egyptian blue, han purple, han blue,lithopone, manganese violet, Prussian blue, ultramarine, venetianceruse, verdigris, vermilion, and viridian

In particular embodiments, the inorganic pigment may be a metal oxide.In some such embodiments, the metal oxide may be an oxide of titanium,such as titanium dioxide. Such titanium based pigments are commonlyemployed in white solder mask base materials. In embodiments, whitesolder base mask materials unexpectedly perform better than green solderbase mask materials, though green solder base mask materials may also beused. In embodiments, the metal oxide is an oxide of titanium, antimonytrioxide, chromium oxide, manganese dioxide, or lead oxide, orcombinations thereof.

Metal oxide based pigments, in particular, exhibit an unexpectedly goodperformance in the cured solder masks. Without being bound by theory,this may be due to the interactions between the metal and thepropylene-glycol-based solvents. Combinations of such pigments andsolvents appear to be at least partially responsible for the observedshear thinning index. Thus, white pigment based on titanium dioxideprovides low shear thinning index as demonstrated below in the Examples.The same level of performance enhancement is not observed for a typicalgreen solder base material, even where the paste-like carrier vehiclefor the pigment is the same. Thus, the carrier vehicle itself does notappear to be the determinative factor in the resultant shear thinningindex.

In embodiments, there are provided methods comprising aerosol jetprinting a solder mask ink in a pattern on a substrate, the solder maskink comprising: i) a resin or UV curable monomer; ii) an inorganicpigment; and iii) from about 20 to about 50 percent by total weight ofthe ink of an ether or an ester solvent based on propylene glycol;wherein the solder mask ink has a viscosity from about 50 cps to about800 cps at shear rate of 10 1/s at 25° C., and a shearing thinning indexfrom about 1.0 to about 1.5; and curing the jetted solder. Inembodiments, the curing step is a thermal of UV curing step.

In some embodiments, solder mask inks herein, when cured may exhibit afilm integrity, as measured by the industrial standard set forth inIPC-SM-840C and its amendment. Some non-limiting exemplary properties ofthe cured solder masks include pencil hardness, dimensional stability,adhesion, chemical resistance, flammability, solderability.

Pencil Hardness:

This test is designed to evaluate the hardness of the solder masksurface and its resistance to abrasion. The test is carried out on threeIPC-B-25A boards coated with solder mask and cured according to themanufacture's specified application and curing requirements. The boardis placed on a firm horizontal surface. The hardest pencil (EagleTurquoise brand ranging from 6H to 4B) is selected and is held firmlyagainst the solder mask at a 45 degree angle. The pencil is then pushedaway from the operator with uniform downward and forward pressure in a ¼inch stroke. If the solder mask is cut or gouged then the next softestpencil is used until one is found which will not cut into the mask. Thepencil hardness is then recorded which did not cut or gouge the soldermask. In embodiments, the cured solder masks herein exhibit a scratchresistance 4-5B and pencil hardness 5-6H.

Embodiments disclosed herein also provide, in part, printed circuitboards comprising conductive patterns disposed on an insulatingsubstrate and a cured solder mask disposed on at least a portion of theconductive pattern, wherein the cured solder mask is formed from asolder mask inks described herein above.

Printed circuit boards may be manufactured by conventional techniquesand may include glass as the insulating substrate over which a copperlaminate sheet is disposed. In some embodiments, the insulatingsubstrate may comprise a rigid or flexible structure. In someembodiments, the insulating substrate is one selected from a glass or aplastic resin.

In embodiments, the conductive pattern may be formed directly on theinsulating substrate. In other embodiments, a solder mask may bedisposed on the copper laminate sheet and the conductive pattern formedon top of the solder mask. In embodiments, a conductive pattern may bedisposed on one or both sides of an insulating substrate and in eithercase a solder mask may be disposed on one or both sides of theinsulating substrate.

In some embodiments, the conductive pattern of a printed circuit boardmay itself be provided by way of a conductive ink. In such embodiments,the conductive ink may be disposed directly on the insulating substrateobviating the need to etch a conductive copper-based pattern from acopper laminate sheet. Where conductive inks are employed, theconductive ink may be disposed on the substrate with the aid of aninkjet printer and subsequently, the solder mask may be printed over theconductive ink. Conductive inks generally comprise conductive particlesdispersed in a carrier fluid. For example, silver nanoparticles andother organic-stabilized metal nanoparticles disclosed in U.S. PatentApplication No. 2011/0305821, which is incorporated herein by referencein its entirety, may be employed in such conductive inks.

PCBs intended for challenging environments may further comprise aconformal coating which is applied by dipping or spraying after thecomponents have been soldered. In some embodiments, such coatings mayprevent, inter alia, corrosion and leakage currents or shorting due tocondensation. In some embodiments, the conformal coating comprises atleast one of a wax, a silicone rubber, a polyurethane, an acrylic resin,and an epoxy resin. PCBs may further be configured with protectiveantistatic agents.

Embodiments disclosed herein also provide, in part, methods of printingsolder masks comprising incorporating a solder mask ink into an aerosoljet printing apparatus, causing droplets of melted solder mask ink to beejected onto a printed circuit board to form a patterned solder mask onthe printed circuit board; and curing the patterned solder mask.

In some such embodiments the curing step may be catalyzed by one or morephotoinitiators present in the solder mask ink. Thus, after printing ofthe solder mask ink, the patterned mask may be cured by exposure tolight, such as UV light. In some embodiments, light curing may beperformed with over a large spectrum of light including UV, IR, near-IR,and visible light. In embodiments, thermal curing is used in lieu of UVcuring.

In embodiments, printing methods may include printing of a conductivepattern with a conductive ink prior to printing the solder mask. In someembodiments, printing solder masks may be performed substantiallysimultaneously with printing of the conductive pattern. This may beachieved, for example, with a tandem printhead with multiple resevoirsthat could alternate between printing insulator and conductor on thesame substrate.

The following Examples are being submitted to illustrate embodiments ofthe present disclosure. These Examples are intended to be illustrativeonly and are not intended to limit the scope of the present disclosure.Also, parts and percentages are by weight unless otherwise indicated. Asused herein, “room temperature” refers to a temperature of from about20° C. to about 25° C.

EXAMPLES Example 1

This example describes the preparation and testing of solder mask inkssuitable for aerosol jet printing application.

Commercial white solder mask resist (Taiyo PSR-4000 LEW1) was purchasedfrom Taiyo America Inc. This solder mask paste is UV curable, butthermally curable materials could be used as well.

Control Example A

The commercial base solder mask paste was characterized first. It showeda viscosity of 35,018 cps at a low shear rate of 10 1/s, and 7490 cps ata high shear rate of 484 1/s at 25° C. The shear thinning index, whichis the ratio of the viscosity at low shear to the viscosity at highshear, was calculated to be 4.68. The viscosity is too high for aerosoljet printing. In accordance with embodiments herein, this commercialsolder mask was modified to achieve a target viscosity less than 1,000cps at the low shear rate of 10 1/s at 25° C. by using the enumeratedsolvents disclosed herein. As the commercial paste has a high shearthinning index, it was desired to identify solvents that can not onlydecrease the viscosity of the original solder mask formulation, but alsoreduce the shear thinning index so that the solvents effectively reducethe viscosity down at low shear rate below 1,000 cps.

The commercial solder mask paste was coated into thin film. After UVcuring with the recommended conditions, the scratch resistance wasmeasured to be 4-5 B, and the pencil hardness was 6H which is thehighest rating for solder mask. In formulating the commercial paste intocomposition for aerosol jet printing the final ink ideally has no sideeffects on the final cured properties such as scratch resistance, pencilhardness, and the like. Moreover, the inks should facilitate formationof aerosols for printing, for example, ideally exhibiting a relativelyhigh boiling point and low vapor pressure.

Control Example B

Following designs commonly used to formulate green solder masks, thewhite solder mask paste employed above was first diluted with alcoholsolvent butyl carbitol, in combination with a small amount of non-ionicsurfactant (Synperonic F 108). Upon adding 25 weight percent butylcarbitol, the viscosity decreased dramatically to 436 and 555 cps at theshear rate of 484 and 10 1/s, respectively. The composition was firstcoated on a copper substrate to test the adhesion and hardness. After UVcuring, scratch resistance of 3-4B and pencil hardness of 2-3H wereobserved, significantly worse than Control Example 1.

Working Example 1

Because solvent can be evaporated substantially completely, it waspostulated that addition of the non-ionic surfactant, Synperonic F108,in Control Example B led to deterioration of the pencil hardness of thefinal cured solder mask. In the course for searching aerosol compatiblesolder mask inks, a solvent only formulation was sought. It wasunexpectedly found that ether and ester solvents based on propyleneglycol provided excellent performance in aerosol jet-compatible soldermask inks. FIG. 2 shows the viscosity (at 25° C.) and shear thinningindex of the new white solder mask ink as a function of the amount ofpropylene glycol monomethyl ether acetate (PGMEA) and dipropylene glycolmonomethyl ether acetate (DPGMEA) solvent mixture (PGMEA and DPGMEA areat 1:1 ratio). This solvent mixture decreased the viscosity veryeffectively. At 25 weight percent solvent, the viscosity decreaseddramatically to 179 and 214 cps at the shear rate of 484 and 10 1/s,respectively, significantly lower than the Control Example B. Moreimportantly, the shear thinning index was reduced dramatically, from4.68 for the commercial solder mask to 1.1-1.2. This helped to reducethe viscosity at low shear rate rapidly.

After adding the solvent, the low viscosity formulation was first coatedon copper cladded FR-4 substrate to test the film forming property. FIG.3 shows the coated film with a formulation having 30 weight percentPGMEA/DPGMEA. A smooth film was observed with excellent wettingproperties on the copper surface. After UV curing, scratch resistance of5B and pencil hardness of 6H were observed, which results are similar tothose of the undiluted commercial paste (Control Example A). The resultsindicate that the use of solvent mixture of PGMEA/DPGMEA had no adverseeffect on the properties of the final cured solder mask. Table 1 belowsummarizes the difference among different formulations, includingscratch resistance and pencil hardness of the white solder mask inkcured coating of the different formulations.

TABLE 1 Scratch Pencil Samples Solvent Addition Resistance HardnessControl A None 4-5 B 6H Control B Butyl carbitol 3-4 B 2-3H WorkingExample1 PGMEA/DPGMEA 5 B 6H

Working Example 2

The formulation in Working Example 1 was printed with an aerosol printerequipped with a pneumatic atomizer at about 50° C. The atomization gaswas set at 1,000 to 1,300 Standard Cubic Centimeters per Minute (SCCM),the exhaust was at 900 to 1,200 SCCM and the sheath gas was at 200 to600 SCCM. Aerosol was generated at such printing conditions via amist-generating probe that was refined with a virtual impactor. The miststeam was then focused using a flow of nitrogen gas. The ink was printedon both PET and copper cladded FR-4 (typical substrate for PCB)substrates. FIG. 4 shows solder mask lines printed at different speedfrom 5.0 mm/s to 20 mm/s. Uniform lines were observed with well-definedline edges. The printed lines showed excellent adhesion, scratchresistance and chemical resistance as was observed above.

It should be noted that aerosol jettable solder mask inks can beformulated directly from epoxy resins, pigments, and solvents, withentirely optional surfactants, adhesion promoters, and/or otheradditives. Thus, the working Examples herein are not so limited to theuse of commercial solder mask as the starting base materials, althoughthey form a very convenient starting point.

1. A solder mask ink for aerosol jet printing comprising a metal oxideand a propylene glycol-based solvent, wherein the solder mask ink has aviscosity from about 50 cps to about 1,000 cps at shear rate of 10 1/sat 25° C., and a shear thinning index from about 1.0 to about 2.0. 2.The solder mask ink of claim 1, further comprising a resin or UV curablemonomer.
 3. The solder mask of claim 2, wherein the resin or UV curablemonomer is selected from the group consisting of a bisphenol A epoxy, anovolak epoxy, an acrylic acid modified epoxy, a cycloaliphatic- orheterocycle-based epoxy, and combinations thereof, the resin beingcrosslinkable with a phenol, amine, or anhydride.
 4. The solder mask inkof claim 1, wherein the metal oxide is an inorganic pigment.
 5. Thesolder mask ink of claim 1, wherein the metal oxide is an oxide oftitanium, antimony trioxide, chromium oxide, manganese dioxide, or leadoxide.
 6. The solder mask ink of claim 1, wherein the solvent is anether and/or an ester solvent based on propylene glycol.
 7. The soldermask ink of claim 6, wherein the solvent is propylene glycol monomethylether acetate, dipropylene glycol monomethyl ether acetate orcombinations thereof.
 8. The solder mask ink of claim 7, wherein thesolvent is a 1:1 combination of solvents propylene glycol monomethylether acetate and dipropylene glycol monomethyl ether acetate.
 9. Thesolder mask ink of claim 8, wherein the 1:1 combination solventscomprises about 10 to about 35 weight percent of the solder mask ink.10. A solder mask ink comprising: i) a resin or UV curable monomer; ii)an inorganic pigment; and iii) from about 20 to about 50 percent bytotal weight of the ink of an ether or an ester solvent based onpropylene glycol; wherein the solder mask ink formulation has aviscosity from about 50 cps about 800 cps at shear rate of 10 1/s at 25°C., and a shearing thinning index from about 1.0 to about 1.5.
 11. Thesolder mask ink of claim 10, wherein the inorganic pigment is selectedfrom the group consisting of a metal oxide, metal sulfate, metalsulfide, metal borate, azurite, aureolin, cobalt blue, chrome yellow,cerulean, calcium chromate, bice cobalt green, Egyptian blue, hanpurple, han blue, lithopone, manganese violet, Prussian blue,ultramarine, venetian ceruse, verdigris, vermilion, and viridian. 12.The solder mask ink of claim 10, wherein the solvent is propylene glycolmonomethyl ether acetate, dipropylene glycol monomethyl ether acetate orcombinations thereof.
 13. The solder mask ink of claim 12, wherein thesolvent is a 1:1 combination of solvents propylene glycol monomethylether acetate and dipropylene glycol monomethyl ether acetate.
 14. Thesolder mask ink of claim 13, wherein the 1:1 combination solventscomprises about 20 to about 35 weight percent of the solder mask ink.15. The solder mask ink of claim 10, wherein the inorganic pigment is ametal oxide.
 16. The solder mask in of claim 15, where the metal oxideis an oxide of titanium, antimony trioxide, chromium oxide, manganesedioxide, or lead oxide.
 17. A method comprising: aerosol jet printing asolder mask ink in a pattern on a substrate, the solder mask inkcomprising: i) a resin or UV curable monomer; ii) an inorganic pigment;and iii) about 20 to about 50 percent by weight of an ether or an estersolvent based on propylene glycol; wherein the solder mask ink has aviscosity from about 50 cps to about 800 cps at shear rate of 10 1/s at25° C., and a shearing thinning index from about 1 to about 1.5; andcuring the jetted solder.
 18. The method of claim 17, wherein the curingstep is a thermal of UV curing step.
 19. The method of claim 17, whereinthe inorganic pigment comprises titanium dioxide.
 20. The method ofclaim 17 having a scratch resistance 4-5B and pencil hardness 5-6H.